Sn1 alkylation of DNA produces many molecular lesions. Among these in relatively high yields are products due to alkylation at the 06 position of guanine, which are mutagenic and very probably carcinogenic. About 20% (now 21 of 106) of human tumor cell lines as well as 8 of 12 SV40 transformed human fibroblast lines (but not non-transformed cell strains from genetically normal people) were discovered in our laboratory to lack mechanisms that repair one such lesion, 06-methylguanine. Such repair-deficient lines have several characterized phenotypic differences from repair-proficient ones, including differential sensitivity to reproductive inactivation by chemotherapeutically used chloroethylating agents. These findings have stimulated interest in understanding the relationship of improper repair of this lesion to both cancer cause and cure. While repair-proficient cells demethylate 06- methylguanine, repair defective cells respond to 06-methylguanine by incorporation of nucleotides without removing the altered base. This repair-like response in repair defective cells may be related to the abnormally high levels of sister-chromatid exchanges, mutagenic, and lethal events observed in repair deficient cells. The studies proposed here would further our understanding of the DNA repair-like events seen in repair deficient cells after treatment with methylating agents. The frequency of such events as well as the frequency of the various methylated bases would be related quantitatively to the production of such endpoints as sister-chromatid exchanges and mutagenic events on a per cell basis in attempts to clarify the molecular relationships between the events. In addition, we wish to resolve conflicting data obtained with cellular hybrids concerning the dominance or recessiveness of the repair-defective phenotype by measuring such dominance or recessiveness in heterokaryons. Our immediate goals then are (1) to determine whether the repair-like incorporation occurs repeatedly over the same DNA regions or more randomly throughout the genome and to determine into which strand (damaged or undamaged) the 06-methylguanine stimulated incorporation occurs; (2) to determine quantitatively the relationship between 06- methylguanine production and the production of lethal events, sister-chromatid exchanges, and mutagenic events in repair- deficient cells, and (3) to determine the dominance or recessiveness, in heterckaryons, of the repair-deficient phenotype.